When I try to add one of my Yahoo email accounts to my iPhone it says "Server Unavailable Please try again later". But I am able to log in to this Yahoo email account successfully from my iMac via Safari and I am also able to add other Yahoo email accounts to the iPhone. It's an iPhone 4 that was just migrated from iPhone 3GS. I've tried both on an iPhone 4 and 4S. It worked fine on the previous iPhone 3GS.